The difference can be miniscule. The only thing that distinguishes a bad vodka from a good one is that the latter lacks a noxious rubbing alcohol smell. Thanks to a brilliant marketing campaign, Absolut has millions of people convinced that it's the best thing since mother's milk.
And once people have settled on it, their loyalty to it can be fierce. You'll never get them to try something else, so they'll never know that there is A grade stuff out there. The loyal MM drinker will never try VW Lot B, and will live out his life in blissful ignorance.
I've seen too many examples of willfull ignorance to discount it the way you appear to. I recollect, back in the 1980s, being part of a hunting party one member of which, at the evening campfire, pulled out a bottle of "bourbon" that was actually Yukon Jack, a liqueur based on Canadian whisky. Efforts by others in the party to convince him that his bottle was not bourbon only succeeded in bringing him to the edge of completely losing it. We left him alone after that (he didn't get invited along the next year, though).